How does natural selection help explain antibiotic resistance?

In the field of Darwinian medicine, antibiotic resistance is a classic example of natural selection in action.

When an antibiotic is introduced to a population of bacteria, it typically kills the vast majority of the susceptible organisms. However, within that population, rare individuals may possess a mutation that confers resistance to the drug, or they may acquire resistance genes through the transfer of plasmids from other bacteria.123

Because the antibiotic eliminates the non-resistant competitors, these resistant survivors are left with reduced competition for resources, allowing them to multiply and become the dominant strain. This process is accelerated by improper use of medication, such as failing to complete a full course of antibiotics or prescribing them for viral infections, which provides the selective pressure necessary for resistant strains to thrive. To counter this, modern medicine often employs "cocktails" of multiple antibiotics; because the probability of a single bacterium simultaneously possessing mutations for three different drugs is extremely low, this strategy significantly reduces the likelihood of survival for the pathogen.23